How did Hurricane Sandy change Mansion Beach?
Last October, Hurricane Sandy bore down upon the northeast coast with the potential for wreaking great damage. Portions of New Jersey, Staten Island, and others fared the worst, and luckily Block Island received only a glancing blow. The most striking images of that storm that we will all remember include the road damage along Corn Neck Road just north of town, and along Spring Street near Pebbly Beach. Careful observation, though, shows many other significant changes that might otherwise go unnoticed. What follows is a case study based on repeated measurements of Mansion Beach. It turns out that Hurricane Sandy eroded the Mansion Beach dunes by a full 33 feet.
I first visited Block Island in 1978, and have returned practically every year. My wife has been coming to Block Island almost every year since 1956. We co-own the Appleby Family Cottage, which was built in 1968 in Sheep’s Meadow. As a geologist, I have studied many beaches and learned the value of preserving their history by periodically measuring the beach profile — each profile is one snapshot in time. I wish I had begun measuring Mansion Beach back in 1978, but I only began constructing beach profiles there with the help of the family in 2007. Normally we collect data in May when we come to open the cottage, and again in August, when we spend our vacation at Block Island. Consequently, I have precise (plus or minus a few inches) beach profiles, collected two times nearly every year, for the last six years.
As the seasons come and go, the beach responds to both the erosional ravages of winter and the gentle growth that occurs each summer. In some winters the collective result of nor’easters is to remove so much sand that a layer of underlying cobbles is exposed along the upper beach. The missing sand is transported offshore and stored in shallow sand bars, although some of it no doubt gets lost forever to deeper water. Then, during most summers the gentle waves slowly move sand back onto Mansion Beach, eventually covering the cobbles. This is sand that was stored in the offshore sand bars, along with new sand that can be seen eroding from the Clayhead bluffs to the north and that gets transported southward along the beach. Our beach profiles record this evolution, and show that while the beach adjusts from season to season, the scarp separating the vegetated dunes from the beach has been nearly stationary.
This year’s Mansion Beach profile was astonishingly different. The illustration shows beach profiles measured in August, 2012, and May, 2013 (please note that the profile is drawn with vertical exaggeration — the vertical and horizontal scales are not the same). Each profile begins at the same benchmark and ends at the water’s edge. The August 2012 profile was typical for a late summer beach, with a lot of sand above mean high tide, and no rocks exposed. The Memorial Day (May 27) 2013 profile is a deeply eroded one, with about 20 meters of exposed rocks (mostly cobbles with smaller pebbles toward lower beach) and a large volume of “missing” sand both from the dunes and from the lower beach near the water’s edge. The profiles show that the scarp retreated 10 meters, or about 33 feet, from its former location.
These two profiles show that the bluff retreated sometime between the two measurements in August 2012 and May 2013. Reports from our neighbors in Sheep’s Meadow, though, indicate that our footpath to Mansion Beach, which was formerly gently sloped, was transformed into “a sharp 10-foot drop” during Hurricane Sandy. We conclude that the storm excavated a huge amount of previously constructed dune material, without much regard to the stabilizing blanket of dune grass that we hoped would help make the dunes stronger.
A major question is whether the dunes will rebuild seaward, or whether this year’s loss of seashore is irreversible. No one can predict for sure, but with a steady rise in sea level (at a rate of about 1 foot per century here in Rhode Island), even if there is a steady supply of new sand coming from the Clayhead bluffs, the prospects are that Mansion Beach may need to adjust to a new normal. And with each major storm or storm season, it may have to do so again and again.
Measuring a beach profile
One of the simplest ways to record the shape of a beach, and to monitor how the beach changes with time, is to make repeated measurements of the beach profile. This involves picking a fixed inland point (like a tree) and surveying a straight line across the beach toward the water. Measurements of horizontal distance and vertical-elevation change can be made with a surveyor’s transit (a telescope-like instrument that is usually attached to a bright yellow tripod), but this method involves expensive equipment. A simpler way is the Emery method, named for the late K. O. Emery of the Woods Hole Oceanographic Institute.
Measuring a beach profile with the Emery method involves two people each with a meter stick, a yard stick, or some other standard measuring stick. At Block Island, we measured beach profiles with sticks that are 1.4 meters long. Beginning at the fixed point (benchmark), one person places a stick on the ground, straight up, and the other person measures 1.4 meters toward the sea and places a stick straight up at that point. Now, this second spot will either be higher, lower, or the same elevation as the first spot. The way to determine which is the case is to imagine a perfectly horizontal line through the top of the lower stick, and measure how much the higher stick pokes up above the line. The two beach profilers accomplish this in an ingenious way: the one with the first stick crouches down to line up the ocean horizon with the two sticks. The line from that person’s eye to the distant horizon is horizontal. By sighting the horizon, the person can read off how much higher or lower the second stick is, and this value is recorded in a notebook.
After the measurement is taken (horizontal distance, and elevation change), the inshore person moves up to 1.4 meters beyond the other person, places the stick straight up, and the new inshore person makes a reading of the next elevation change. In this way, by taking turns and moving 1.4 meters closer to the water with each measurement, the pair can collect and record the data needed to plot a beach profile — it only takes about 15 or 20 minutes. The measurements are easily entered into a computer spreadsheet and plotted (see the figure accompanying the article).
David A. Vanko, Ph.D. is the Dean of the Jess and Mildred Fisher College of Science and Mathematics at Towson University. He is a geologist who researches deep-sea volcanism and hydrothermal systems. Vanko speculates that sea level at Block Island will rise between about 40 and 80 cm by the year 2100.